Fish, fish oil, and eggs have been identified as the top three sources of certain industrial toxins in the diet (see also here). Factory farmed fish have been found to have the highest levels of DDT, PCBs, and dioxins (compared to wild caught fish). Tuna has been found to be especially high in mercury (see here, here). But all types of fish have been found to be contaminated with industrial toxins and, thus, microalgae based DHA is a better option for long chain omega-3s (see also here).